(a) Field of the Invention
The present invention relates to an optically compensated bend (OCB) mode LCD device and, more particularly, to an OCB mode LCD device having an improved contrast ratio.
(b) Description of the Related Art
LCD devices are increasingly used in recent days while replacing the conventional CRT display device due to its advantages of smaller thickness, feasibility of larger capacity for display data etc.
A twisted nematic (TN) mode is generally employed as the operational mode of the LCD device. The TN mode is such that the direction of the axes (sometimes referred to as xe2x80x9cdirectorsxe2x80x9d hereinafter) of the LC molecules are twist-rotated by 90 degrees in the direction perpendicular to the substrate surface between the front substrate and the rear substrate by using a perpendicular electric field which is normal to the substrate surface.
The TN mode has a disadvantage, however, in that the resultant LCD device has a narrow viewing angle, which hinders the picture on the screen from being clearly observed in the diagonal direction with respect to the perpendicular of the screen (substrate surface). In addition, in the case of a larger screen display device, a picture element appearing on the center of the screen and another picture element appearing on the periphery of the screen provide different image characteristics as observed from a point diagonally with respect to the perpendicular of the screen, whereby a correct image display is not possible.
JP-A-6-75116 describes a TN mode LCD device wherein a phase compensating plate is provided for enlarging the viewing angle. However, even this technique cannot solve the above problem to a desired extent or sufficiently compensate the twisted structure peculiar to the TN mode LCD device.
There is another technique of interest for improving the narrow viewing angle, called OCB mode wherein a LC cell having a bend orientation arrangement (or may be a parallel orientation arrangement) is combined with a phase compensating plate. The OCB mode is especially noticed due to its higher-speed response.
FIG. 1 shows a schematic chart showing orientation arrangements of directors of the LC molecules between the substrates, including a splay orientation arrangement, a twist orientation arrangement and a bend orientation arrangement, as viewed from the left of the drawing. Of these orientation arrangements, the bend orientation arrangement has a plane symmetry structure with respect to the central plane between the substrates, wherein the directors of the LC molecules reside in a standing posture, or are normal to the substrate surface, at the central area and xe2x80x9cfallsxe2x80x9d toward both the substrates to be parallel to the substrate surfaces in the vicinity of the substrates.
The OCB mode is achieved by providing a LC layer having a bend orientation arrangement between the substrates and a phase compensating plate or plates for compensating the phase of the LC layer.
Known techniques for using the phase compensating plate in the LC device having the bend orientation arrangement include one using a phase compensating plate having a negative birefringence as described in JP-A-6-294962, one using a bi-axial phase compensating plate as described by Kuo in SIDxe2x80x2 94 Digest, and one using a pair of phase compensating plates each having a negative birefringence and a hybrid orientation arrangement as described in JP-A-10-197862.
FIG. 2 shows the structure of the conventional OCB mode LCD device described in JP-A-10-197862, as mentioned above. A first substrate 21 mounts thereon either red, green or blue color filter 29R, 29G or 29B for each pixel area, on which an overcoat film 13, a common electrode 10 and a first orientation film 15 are consecutively formed.
A second substrate 22 mounts thereon a pixel electrode 27R, 27G and 27B either for red, green and blue color for each pixel area, on which a second orientation film 16 is formed.
First and second substrates 21 and 22 oppose each other with a LC layer 23 being sandwiched therebetween. On the outer surface of the first substrate 21, a first phase compensating plate 24 and a first polarizing plate 11 are consecutively formed. On the outer surface of the second substrate 22, a second phase compensating plate 25 and a second polarizing plate 12 are consecutively formed.
FIG. 3 shows orientations of the LC layer together with the axes of the polarizing plates 11 and 12 and the phase compensating plates 24 and 25, as viewed from the first substrate side. FIG. 4 shows schematic sectional view depicting the directors or the LC molecules and birefringences of the phase compensating plates as well as birefringence eclipses of the LC layer and the phase compensating plates during displaying a black color, wherein xe2x80x9cnexe2x80x9d denotes the abnormal optical axis and xe2x80x9cnoxe2x80x9d denotes a normal optical axis.
In FIG. 3, the orientations 101 and 102 of the first and second orientation films are formed in the same direction as the inclined directions 201 and 202 of the birefringences of the phase compensating plates so that the abnormal optical axis xe2x80x9cnexe2x80x9d of the LC layer resides in the same direction as the abnormal optical axis xe2x80x9cnexe2x80x9d of the birefringence of the phase compensating plates.
The polarizing axis 301 of the first polarizing plate is set at 45 degrees away from the orientation 101 of the first orientation film, and the polarizing axis 302 of the second polarizing plate is set at 90 degrees away from the polarizing axis of the first polarizing plate.
In FIG. 4, symbols LC1 to LC5 show a birefringence eclipse at the respective divided planes of the LC layer divided into ten layers, whereas symbols RF1 to RF5 show a birefringence eclipse at the respective divided planes of the phase compensating plate divided into five layers. In this example, it is assumed for simplicity that each layer has an equal thickness.
The longer axis of the birefringence LC1 in the central area of the LC layer is normal to the substrate surface, and the longer axis of the birefringence LC5 of the LC layer in the vicinity of the orientation layer is parallel to the substrate surface, with the axes of the birefringences having intermediate numbers LC2, LC3, LC4 resides between those directions. On the other hand, the longer axis of the birefringence RF1 of the phase compensating plate at the outer surface thereof is normal to the substrate surface and the longer axis of the birefringence RF5 at the inner surface is parallel to the substrate surface.
The negative birefringence of the phase compensating plate oriented in a hybrid orientation arrangement corresponds to the birefringence of the LC layer when the LC layer displays a black color. The birefringences LC1, LC2, . . . , LC5 of the LC layer correspond to the birefringences RF1, RF2, . . . , RF5, respectively, of the phase compensating plate for effecting compensation of retardation.
The overall retardation xe2x80x9cRxe2x80x9d of the LCD device can be expressed by equation (1) based on the refractive indices and the thicknesses of the LC layer and the phase compensating plate:
R=Rlc+Rrf=[(nlcxxc3x97dlc+nrfxxc3x97drf)xe2x88x92(nlcyxc3x97dlc+nrfyxc3x97drf)]xe2x80x83xe2x80x83(1)
wherein nlcx, nlcy, nrfx and nrfy are the refractive indices of the LC layer in x-direction, the LC layer in y-direction, the phase compensating plate in x-direction and the phase compensating plate in y-direction, respectively, all of these being observed from a single point, and wherein dlc and drf are the thicknesses of the LC layer and the phase compensating plate, respectively.
Rlc and Rrf are retardations of the LC layer and the phase compensating plate, and are expressed by;
Rlc=(nlcxxe2x88x92nlcy)xc3x97dlc, and
Rrf=(nrfxxe2x88x92nrfy)xc3x97drf.
If the birefringences LC5 and RF5, for example, are observed from the front, birefringence LC5 has a larger refractive index in x-direction whereas birefringence RF5 has a larger refractive index in y-direction, as shown in FIG. 5A. Thus, both the birefringences LC5 and RF5 compensate one another to obtain an equal refractive index for x-direction and y-direction as a whole, thereby providing substantially zero for the retardation.
On the other hand, if the birefringences LC5 and RP5 are observed diagonally along the rubbing direction of the orientation film, as shown in FIG. 5B, the birefringence LC5 is smaller than that observed from the front, whereas the birefringence RF5 is larger than that observed from the front. Thus, both the birefringences LC5 and RF5 as observed diagonally also compensate one another to thereby provide substantially zero for retardation.
As in the case of the combination of birefringences LC5 and RF5, other combinations LC1 and RF, LC2 and RF2, . . . also compensate one another, whereby the OCB mode LCD device provides substantially zero for retardation to display a correct black color even viewed diagonally and thus provides a wider viewing angle.
The electric characteristics of the OCB mode LCD device will be described hereinafter. It is assumed that the product xcex94nxc3x97d of the birefringence factor xcex94n and the cell thickness xe2x80x9cdxe2x80x9d is between 790 nm and 1190 nm, as described in JP-A-10-197862. This product range occurs when all the LCD molecules reside parallel to the substrate surface. On the other hand, the bend orientation arrangement occurs when all the LC molecules in the central area are normal to the substrate surface, whereby the retardation Rlc of the LC layer is ⅓ to xc2xd to that range. The retardation Rrf of the phase compensating plate resides at 20 to 50 nm as viewed from the front in consideration of the retardation of the commercial phase compensating plate having a negative birefringence and a hybrid orientation arrangement. The retardation Rrf as used herein means such provided by a separate phase compensating plate.
Since the longer axis for the birefringence of the phase compensating plate is normal to the longer axis for the birefringence of the LC molecules, as shown in FIGS. 3 and 4, the retardation of the phase compensating plate acts in a negative direction ,with the longer axis for the birefringence of the LC molecules being in the positive direction, as will be understood from equation (1). Thus, the overall retardation xe2x80x9cRxe2x80x9d of the LCD device resides between 250 nm and 300 nm when the LCD device exhibits a white color.
In a LCD device using the birefringence, the intensity of transmitted light xe2x80x9cIxe2x80x9d is represented by:
I=Axc3x97(sin(2xcex8))2xc3x97sin(Rxcfx80/xcex))2xe2x80x83xe2x80x83(2)
wherein A, xcex8, R and xcex are a constant, the angle between the polarizing axis of the polarizing plate and the axis of LC layer, the overall retardation and the wavelength of transmitted light.
With the increase of the voltage applied to the LC layer to thereby decrease the retardation of the LC layer, the overall retardation becomes small and eventually become zero at a higher applied voltage whereby a black color is displayed.
In the conventional OCB mode LCD device, there is a drawback especially in displaying a color image, as will be detailed below. If the overall retardation R is zero or an integral multiple of the wavelength of the transmitted light, the LCD device assumes a black color. However, the conventional LCD device has a higher brightness during displaying a black color whereby a contrast ratio is reduced, as detailed below.
Both the retardations of the LC layer and the phase compensating plate are not constants for different wavelengths, i.e., have wavelength dependencies. For example, if a LC cell having a bend orientation arrangement is formed by using the LC material and the conditions such as shown in table 1, the overall retardation exhibits a wavelength dependency and an applied-voltage dependency, such as shown in FIG. 6.
wherein k11, k22 and k33 are splay viscosity/elasticity factor, twist viscosity/elasticity factor and bend viscosity/elasticity factor, respectively.
On the other hand, the phase compensating plate having a negative birefringence and a hybrid orientation arrangement exhibits a wavelength dependency of the retardation as shown in FIG. 7. A pair of such phase compensating plates each having the characteristic of FIG. 7 exhibit an overall retardation in the OCB mode LCD device, such as shown in FIG. 8.
In this case, an applied voltage of 7 volts provides a minimum brightness to exhibit a black color. The minimum brightness is 1.5 folds the brightness provided by a conventional TN mode LCD device including a phase compensating plate having a negative birefringence and a hybrid orientation arrangement, thereby exhibiting a poor contrast. This is because the overall retardation is large and light leaks in the wavelength bands between 400 nm and 500 nm and between 600 nm and 800 nm.
The TN mode LCD device including a phase compensating plate having a negative birefringence and a hybrid orientation arrangement does not exhibit such a phenomenon or a poor contrast.
FIG. 9 shows a schematic chart showing birefringences of the LC layer and the phase compensating plate in a TN mode LCD device exhibiting a black color, whereas FIG. 10 shows the directions of the axes of the LC layer, phase compensating plate and the polarizing plate in the TN mode LCD device. FIG. 11 is a schematic chart of birefringence eclipse as viewed from the front of the TN mode LCD device, wherein the horizontal axis 501 and the vertical axis 502 are transmission axes of the polarizing plates, symbol RF corresponds to the birefringence eclipse of the phase compensating plate and symbol LC corresponds to the birefringence eclipse of the LC layer.
The light which is incident on the TN mode LCD device passes the phase compensating plate without the influence by the birefringence thereof to all the wavelengths of light because the main axis of the birefringence of the phase compensating plate disposed on the incident side is aligned with the polarizing axis of the polarizing plate. That is, the plane (or linearly) polarized light aligned with the polarizing axis of the polarizing plate on the incident side reaches the TN cell as it is. If the LC molecules are applied with a suitable high voltage to xe2x80x9cstandxe2x80x9d, the light passes the LC layer to reach the phase compensating plate on the outgoing side (opposite side) without generating a phase difference between wavelengths. Since the birefringence axis of the phase compensating plate on the opposite side is normal to the polarizing axis of the polarizing plate on the incident side, the light also passes the phase compensating plate on the opposite side without the influence by the birefringence thereof. Thus, the incident light passes the polarizing plate on the opposite side as the original plane polarized light irrespective of the wavelength thereof, whereby a black color is displayed on the LCD device.
FIG. 12 shows a schematic chart of the birefringence of the OCB mode LCD device, wherein numerals and symbols similar to FIG. 11 are used to designate similar elements. Since the polarizing axis 601 of the polarizing plate on the incident side and the birefringence axis of the phase compensating plate have a specified angle (45 degrees in this case) therebetween, the light is affected by the birefringence for any color to be displayed, whereby the wavelength dependency of the birefringence affects the display. This problem is common to a LCD device including a phase compensating plate having a hybrid orientation arrangement and a negative birefringence and a LCD device including a phase compensating plate having a bi-axial birefringence.
FIG. 13 shows a schematic chart of the birefringences of the LC layer and the phase compensating plate in a LCD device including a LC cell having a parallel orientation arrangement. This chart is similar to the chart of FIG. 4 except that a point symmetry arrangement is employed in FIG. 13 with respect to the center of the LC layer instead of the plane symmetry arrangement, whereby the LCD device also suffers from a similar problem.
It is therefore an object of the present invention to provide a OCB mode LCD device capable of solving the above problem of poor contrast.
The present invention provides a LCD device including first and second substrates opposing each other, a LC layer sandwiched therebetween for allowing transmission of light therethrough, an electrode assembly for defining a plurality of pixel areas in the LC layer, a phase compensating plate mounted by either one of the first and second substrates, a wavelength-dependent compensating plate mounted by either one of the first and second substrates, the wavelength compensating plate compensating a wavelength dependency of an overall retardation effected by the LC layer and the phase compensating plate to thereby obtain a substantially constant retardation with respect to different wavelengths of the transmitted light.
In accordance with the LCD device of the present invention, since the wavelength dependency of the overall retardation of the LCD device is compensated by the wavelength-dependent compensating plate, the resultant LCD device has a higher contrast ratio, thereby improving the image quality of the LCD device.
In the above configuration, both the first and second substrates may mount thereon respective phase compensating plates, and also may mount thereon respective wavelength-dependent compensating plates independently of the phase compensating plates. In addition, a plurality of phase compensating plates may be mounted by one of the first and second substrates.